The present invention relates to the field of image sensing and more specifically to a vertically integrated complementary metal oxide semiconductor (CMOS) image sensor.
Solid state image sensors, which are used in applications such as digital cameras, are currently available in numerous forms. Charge coupled devices (CCD) and CMOS image sensors, for example, are based on a two dimensional array of pixels. Each pixel contains a light sensing device that is capable of converting an optical image into an electronic signal. When photons strike the photoactive region of the array, free charge carriers are generated in an amount that is linearly proportional to the incident photon radiation. The photon generated charge carriers are collected and moved to output circuitry for processing. The pixels are typically disposed in rows and columns to form the array. The number of pixels in the array determines the resolution of the final image produced by the sensors. Arrays of 640xc3x97480 are used for low-cost digital cameras and high-end digital cameras have a resolution of at least 1,000xc3x97800.
Recent developments in electronic imaging have created the need for image sensors with improved performance and additional functions. Some of these applications require a high dynamic range and on-chip signal processing. Conventional CCDs are unable to achieve these goals due to their limited dynamic range, serial readout mode, and pixels that can only perform limited xe2x80x9cintelligentxe2x80x9d operations. Furthermore, the serial read out scheme for CCDs precludes random access to individual pixels, so that the image processing is limited to the entire image.
To overcome the problems inherent in CCDs, laterally integrated CMOS sensors have been developed using CMOS circuitry in each pixel and photodiodes for the light sensing devices. CMOS image sensors have numerous advantages over conventional CCDs. They scale better than CCDs because as the CMOS process technology evolves, more complex circuitry can be added to each pixel. The additional circuitry increases sensitivity and reduces noise, allowing the pixel to shrink while maintaining the same functionality. CMOS sensors also have a cost advantage over CCDs because they can take advantage of the CMOS fabrication process. The cost of CMOS image sensors is further reduced because the CMOS process is much less complex than the CCD process. Since the CMOS process is less complex, the yield of CMOS image sensor is typically higher. This is partially due to the fact that the sensor does not use a serial read out process, so a defect in the chip will only affect a single pixel.
Typical prior art CMOS image sensors in use rely on a lateral integration of the photodetector and the pixel electronics. While laterally integrated CMOS sensors overcome some of the limitations of CCDs, they also suffer from some disadvantages. In a laterally integrated CMOS image sensor, the photodetector and the CMOS circuitry are fabricated next to each other on the silicon substrate. Thus, additional lateral area (xe2x80x9creal estatexe2x80x9d) is required for the lateral integration of the photodetector with the CMOS circuitry. This can reduce the area fill factor and limit the possible resolution. Furthermore, it is difficult to optimize the process technology for both the CMOS transistor and for the photodiode at the same time. So, while the design of fast CMOS circuits demands the use of shallow junctions with very low sheet resistance, this is totally inconsistent with using these junctions as a photodiode. Also, when additional on-chip functions are added to the sensors, either the pixel size will increase to maintain the sensitivity of the sensor or the area required for the photodiode will decrease to maintain the pixel size. If the pixel size increases to maintain the sensitivity, the resolution of the sensor will decrease. If the photodiode area decreases to maintain the pixel size, the sensitivity of the sensor may decrease.
Accordingly, the present invention pertains to the integration of photodetectors with CMOS circuitry.